This invention relates generally to time domain reflectometers, and in particular to a time domain reflectometer having a dual time base to provide high-resolution, zero dead time measurement of reflection characteristics in all lengths of cables.
A time domain reflectometer (TDR) is an instrument which indicates and measures reflection characteristics of a transmission system by launching a signal into the transmission path and measuring the length of time required to receive a reflection from a discontinuity in the transmission path. Electrical TDRs measure and locate faults, such as opens and shorts, in transmission lines such as cables and the like, and optical TDRs similarly measure and locate faults, such as breaks and kinks, in fiber-optic filaments.
Conventional high-resolution TDRs launch a short-duration pulse into a transmission path and measure the length of time it takes to receive a reflection using a high-speed clock and logic circuits. The distance D from the TDR to the fault (or discontinuity caused by the end of a cable) is proportional to one-half the measured time interval t.sub.m between the launched stimulus pulse and a reflected response. That is to say, a pulse launched into a transmission system travels at a propagation velocity v.sub.p, which is a percentage of the speed of light and is approximately equal to eight inches per nanosecond (or 20 centimeters per nanosecond) in an electrical cable, through the transmission system to a discontinuity and reflected therefrom back to the TDR, so that D=v.sub.p .multidot.1/2t.sub.m. Thus, to resolve a measurement of the distance D to a fault in a cable (or the length L of the cable) to within four inches (10 centimeters) using a conventional TDR would require a measurement clock having a frequency of one gigahertz.
As a practical matter, the power requirements and expense of high-speed clocks preclude their use in most commercial TDR equipment, and so clock rates of around 100 megahertz (MHz) are typical, resulting in typical resolutions of from two to four feet. This sort of resolution may be adequate for cables that are several hundred feet in length; however, when cable lengths of six or twelve feet are being measured, a resolution of from two to four feet is inadequate. Also, TDRs having high-speed circuits are very complex and difficult to interpret, are very expensive, and they consume large amounts of power.
Another problem associated with conventional TDRs is that the measurement circuits are not sensitized to reflections for some brief period of time, e.g., several nanoseconds, following launch of a pulse into the transmission path for various reasons. For one thing, some conventional measurement circuits cannot recognize a reflection while the stimulus pulse is being launched. Another reason for blocking immediate reflections in some prior-art TDR systems is to prevent high-energy launch pulses from damaging measurement circuits in some cases. This brief period of time that the measurement circuits are blocked results in what is known in the art as a dead zone or a blind spot in the first few feet of the transmission path. Dead zone is a key specification of most conventional TDRs because it indicates the minimum length of cable that can be measured. Dead zones of 20 feet are typical.
There are many situations in which it would be desirable to measure relatively short cables, such as those found in a local-area network (LAN) for computers. In a LAN, some of the cables interconnecting computers are relatively short, e.g., six feet in length, while other cables routed through walls and ceilings can be relatively long, e.g., 2000 to 4000 feet in length. A TDR would quickly locate faulty connectors, and even determine how much cable is left on a spool. Such a TDR would have to have to be capable of measuring short cables with high resolution, and it would have be relatively portable, exhibit low power consumption for battery operation, and be inexpensive and easy to use.